A power transistor generally has normally-on characteristics with which drain current flows when voltage between a gate and a source is 0 V. Thus, when drain voltage is applied without applying sufficient negative polarity voltage to a gate terminal of the power transistor, large current flows and the power transistor may be broken. Accordingly, the power transistor is more difficult to be handled than normally-off transistors such as a bipolar transistor and a metal-oxide-semiconductor field-effect transistor (MOSFET).
Thus, a switching circuit in which a high-withstand-voltage normally-on transistor and a low-withstand-voltage normally-off transistor are connected in series has been conventionally known. For example, FIG. 4 of PTL 1 illustrates a power conversion device that includes a circuit in which a normally-on junction FET and a MOSFET are connected in series. FIG. 1 of PTL 1 illustrates a power conversion device that includes a diode connected in reverse parallel to a circuit, in which a normally-on transistor and a normally-off transistor are connected in series, and controls gate voltage of the two transistors by using a gate driving circuit. PTL 2 describes a switching power supply device that includes a circuit in which a high-withstand-voltage normally-off transistor and a low-withstand-voltage normally-off transistor are connected in series.